Applications of Outcrop Gamma-Ray Logging to Field Development and Exploration: Reservoir Characterization: Petrophysical Formation Evaluation and Rock Description

Gamma-ray logs of outcrops have been generated using two techniques. These techniques demonstrate the applicability of outcrop logging to better understand reservoir facies architecture and exploration type problems. The first logging technique employs the use of a standard logging truck and gamma-ray sonde. The truck is positioned near the top of the cliff face and the sonde is lowered to the bottom of the cliff. Gamma-ray counts are recorded as the sonde is raised at a constant rate. The second logging technique employs the use of a commercially available, hand-held, gamma-ray scintillometer. The tool measures total radiation at the outcrop. Equally-spaced measurements are made along the section and are displayed as a function of depth below a reference point. Examples of gamma-ray logging experiments conducted on turbidites of the Jackfork Group (Pennsylvanian) in central and southern Arkansas demonstrate: (1) the reliability, potential pitfalls, and degree of uncertainty associated with correlating subsurface gamma-ray logs, (2) the nature of vertical stratification and bed cyclicity in these types of sequences as interpreted from logs, (3) the relation of gamma-ray response to lithology and reservoir quality, and (4) the seismic expression of these types of strata. Perhaps the most powerful application of outcrop gamma-ray logging is the ability to improve interpretations and correlations of subsurface well logs by comparing them with gamma-ray logs obtained from analogous outcrops. Examples from the Long Beach Unit of the Wilmington Oil Field, California, and Point Mugu (Santa Barbara Channel), California demonstrate this application.